The smoke point of a hydrocarbon is a characteristic that is routinely determined in the laboratories of refineries, mainly on kerosenes, aviation fuels and even lamp oils.
This characteristic is an important parameter since it is directly linked to the hydrocarbon composition of the fuels under test. In practice, the greater the C/H ratio, and therefore the lower the aromatic compound content, the higher the smoke point becomes and the better the fuel behaves on its combustion. In other words, the smoke point is quantitatively linked to the potential transfer of radiative heat and, in as much as this heat transfer exerts a strong influence on the temperature of the metallic parts, the smoke point therefore becomes a predictive indicator of the longevity of said metallic parts.
The smoke point does, unfortunately, however have the drawback of being fairly difficult to measure. Normally, for this, an analysis method is used that is the subject of a standardization (such as the method described in the ASTM D 1322 standard and its equivalents, such as ISO 3014, IP 57 and NF M 07-028) to enable the detection, then the measurement of the maximum height of a flame (normally expressed in mm and accurate to the nearest tenth of an mm) which can be obtained from the hydrocarbon under test without the formation of smoke.
In such a measurement, the sample is burned in a wick lamp, also described in the ASTM D 1322 standard, and the operator varies the position of the burner so as to gradually modify the height and the appearance of the flame, which changes slowly from a relatively elongated and jumpy state with a top end giving off a light smoke, to a state in which the flame height is shorter, with a top end that is perfectly rounded. Between these two flame states, the operator must also distinguish two other intermediate shapes, namely that having an elongated point, the edges of which appear concave in the top part and the one in which the pointed end has just disappeared and which forms a flame that is slightly rounded without smoke. It is when the flame has this last appearance that the operator records the height of the flame, on a scale graduated in mm positioned inside and at the back of the lamp. The final value of the smoke point retained for the sample under analysis is the average of three successive measurements, calculated to the nearest 0.1 mm.
The method of measuring the smoke point, as defined in the ASTM D 1322 standard, like all the analysis methods of this type, does, however, have limitations in terms of accuracy, mainly due to the assessment difficulties of the operator, in particular when taking the decision to judge the correct appearance of the flame, according to the standard, and also at the moment when the height of this flame is measured visually on the graduated scale. In practice, the good quality of the measurement of this height requires particular procedural precautions, the application of which depends entirely on the operator. Thus, the repeatability and reproducibility of the standardized test are respectively 2 mm and 3 mm.
The Applicant has proposed to remedy this difficulty by replacing the eye and the brain of the operator with a technical system or acquiring digital images mainly comprising a digital camera, or an equivalent of such a digital camera, and an associated computer system for analyzing and processing the stored digital images. However, the distinguishing of the different characteristic shapes of the flame, between the relatively elongated and jumpy state with a top end giving off a light smoke and the state presenting a shorter flame height with a top end that is perfectly rounded, is subjective and is ill-suited to the normal use of a computer program for analyzing and processing digital images. The Applicant, after much research work, has found that, surprisingly, the use of such a system associated with a choice of appropriate parameters and a correct calibration, would make it possible to spectacularly increase the accuracy of the method and obtain the smoke point of the hydrocarbon under test but without the problem of the subjectivity of the operator.